Since the discovery of single-walled carbon nanotubes (SWNTs), research on applications of these unique materials has increased at a rapid rate. The large variety of SWNT-based materials parallels the large number of potential commercial applications, which include polymeric composites, field emission displays, electrical capacitors, and thermal management materials. Accordingly, a number of different techniques have been proposed to manufacture SWNT-based materials. The current demand of these materials is mostly related to research activities. Several factors are responsible for the observed delay in transferring these activities from the labs into the commercial applications. First, the high cost of SWNTs at this stage of production has limited the amounts that can be made available for large-scale development. Second, the difficulties in handling and dispersing these materials that are inherent to SWNTs make their incorporation in useful matrices a challenge. The incompatibility of SWNTs with most typical solvents limits their effective handling and widespread use, since, when placed in water or most organic solvents, nanotubes generally quickly fall out of suspension even after strong sonication. Third, due to the extraordinary properties of carbon nanotubes, many new applications are appearing everyday and each application may require a different handling and dispersion procedure. Interestingly, while only a very small amount of nanotubes may be sufficient to achieve greatly improved properties in some applications, in other applications the concentration SWNTs used may need to be much higher.
Even though individual SWNTs have well-defined ideal structures, they bundle-up in ropes of different sizes, depending on the particular methods of synthesis and handling chosen by the manufacturers. The few commercial sources of SWNTs have made their product available in a variety of forms. For example, SWNTs produced by arc discharge and laser ablation have been commercialized as a soot of varying particle sizes containing different concentrations of residual catalyst. In other cases, SWNTs have been commercialized as a suspension in liquid media. The first attempts of solubilizing nanotubes made use of the possibility of shortening the SWNT by acid attack in concentrated sulfuric-nitric mixtures. These aggressive treatments were shown, however, to introduce a significant number of defects in the SWNTs along with amorphous carbon that is generally undesirable. Other products introduced external elements to facilitate the dispersion. They have included chemical modification of the nanotubes and the use of wetting agents such as surfactants.
The most extended functionalization method is the one developed by Haddon et al., in which the nanotubes are dissolved in chloroform, benzene, toluene or other organic solvents after oxidation and subsequent derivatization with thionylchloride and octadecylamine. Alternative approaches make use of the partial oxidation of SWNTs, followed by sidewall reactions with fluorine, alkanes, diazonium salts, or by ionic functionalization. The main disadvantage of these methods is the inevitable distortion of the original structure and chemistry of the nanotubes.
Other groups have opted for the attachment of soluble polymers to SWNTs by various methods. For example, O'Connell et al., developed a non-covalent association of SWNTs with linear polymers such as polyvinyl pyrrolidone and polystyrene sulfonate. The intimate interaction that results between the polymers and the SWNTs result in an increased suspendability of the nanotubes in water. A similar method has been developed at Zyvex, although in this case, as proposed by these inventors, the functionalization does not involve the nanotube wrapping by the polymer, but rather a noncovalent bond between a conjugated polymer and the nanotube. It is proposed that the interaction between the polymer backbone and the nanotubes is due to π-π bonding. Although this particular solubilization seems to be effective for the enhancement of the nanotubes solubility, times of at least 30 minutes are required for the re-dispersion of these materials and a limited number of “selected” organic solvents can be used.
Surfactants have also been extensively used to obtain concentrated nanotube suspensions. Intensive work at various universities has demonstrated the effectiveness of different surfactants in dispersing single-walled nanotube materials. There are a number of publications reporting the use of sodium dodecylsulfate, sodium dodecylbezenesulfonate and TRITON X among others. However, due to the strong Van der Waals interactions between the nanotube surface and the hydrophobic tails of the surfactant molecules, the removal of the surfactant from the nanotubes becomes very problematic. Moreover, the maximum concentrations of SWNTs achieved in these suspensions are exceedingly low.
Smalley et al., have proposed to use alewives as a redispersable SWNT product. This product is produced by treatment in liquid superacids such as oleum (a highly concentrated sulfuric acid) and other corrosive liquids. The main disadvantage of this method is the need to handle hazardous liquids. In addition, this type of aggregate cannot be formed in anhydrous media, whereas the product of the present invention can be made in either aqueous or non-aqueous media as described in more detail below.
It is clearly apparent that a method to produce readily soluble SWNTs would be greatly advantageous for the development of applications of SWNT-based materials. It is to this end that the present invention is directed.